When battery driven refrigeration systems are positioned in vehicles as described above, it will sometimes occur that the engine of the vehicle is not running, but it is necessary to keep the refrigeration system running. In such cases power for the refrigeration system is only available from a limited source, such as the battery of the vehicle. It is of great importance to use as little power as possible in running the refrigeration system so as to ensure as long a period of refrigeration as possible before the battery requires recharging.
Such a refrigeration system may comprise a control unit and a compressor and associated refrigeration circuit for providing refrigeration to a refrigeration compartment. The refrigeration compartment is often an insulated box with a removable or hingeable door or lid to allow occasional access to the contents of the refrigeration compartment. It may further comprise an illumination device (for example an electric lamp) for lighting the inside of the refrigeration compartment.
One way of conserving power is to close down all unnecessary electronic circuits. For example, the power converter circuits which supply power to the compressor could be shut down when the compressor is not required (that is, when further refrigeration is not required).
Such power supplies may also be used to supply power to other accessories, such as a lamp inside the refrigeration compartment. In the case of a lamp it is important that power is available for the lamp when the door is open, even if the compressor is not running.
The standard way of controlling lighting in coolers and fridges is by an electrical switch which closes when the door is opened so that the lamp is only lit when the door is open. Such a lighting system is illustrated in FIG. 1, where a lamp 1 is positioned inside a refrigeration compartment 2. The lamp 1 is connected between a power source (not shown) and ground via a switch 3. The switch 3 is connected to a door or a lid in such a manner that the switch 3 is open when the door or lid is closed (thereby preventing a connection between the power source and the lamp 1), and the switch 3 is closed when the door or lid is open (thereby connecting the power source and the lamp 1). In FIG. 1 the switch 3 is open, and accordingly no power is supplied to the lamp 1.
In order to use the facility described above, the power supply must be running all the time, maintaining a voltage on the lamp supply output, even when the lamp is not in use. Since the power supply circuits use some power in this mode, even if no power is being consumed by the lamp, it is a disadvantage. One possible solution is to use two circuits, one leading to the switch (in order to monitor whether it is open or closed), and a second one leading to the lighting device itself and supplied with power from a power supply which runs only when the door is open. This is illustrated in FIG. 2. This solution is not the optimum since it requires extra wiring (and therefore entails extra cost) and, since it is not an industry standard, requires the manufacturers of refrigeration systems to modify their product to accommodate the new control system.
When the compressor is required (that is, when refrigeration is required) there is little additional cost in running a power supply for lighting the lamp, even if the lamp is not lit. However, when cooling is not required it would be an advantage to monitor the door state whilst using very little power, and without the use of extra circuitry external to the control unit.